Any multicellular animal poses a special difficulty for the theory of evolution. Most of its cells will die without reproducing, and only a privileged few will pass their genes. Given the incentive for cheating, how is cooperation among the cells enforced? In the Dec. 16 issue of the journal Science, Washington University in St. Louis biologists Joan Strassmann and David Queller suggest the answer is frequent population bottlenecks that restart populations from a single cell.